Ceramic Burner Plate

ABSTRACT

The present invention relates to a ceramic burner plate for infrared radiators which includes a lithium silicate as a main constituent. The burner plates according to the invention can have a content of lithium oxide within the range of 0.63% by weight to 7.6% by weight and they exhibit a high temperature resistance and are extremely durable.

The present invention relates to a ceramic burner plate for infrared radiators including lithium silicate as a main constituent.

Ceramic burner plates are used in infrared radiators in which for heat production a gas-oxygen mixture is burnt on the surface of the ceramic plates. During this operation, infrared radiation is produced which is utilized for heat production. The advantage of infrared radiators over conventional heating systems is on one hand that infrared radiators are able to dissipate heat almost without loss, since any carrier medium is not required for the transfer of energy and the heat is dissipated in the form of infrared radiation, and on the other hand that draught phenomena as occurring in conventional combustion systems are avoided.

While the ceramic plates that were formerly used as burner plates had a relatively simple structure, modern ceramic burner plates exhibit complex surface structures, by which the efficiency and the emission behavior can be considerably influenced. Today, a burner plate includes for instance between 3000 and 4000 holes having a diameter from 1 to 1.3 mm. The so-called deepness effect structure of the burner plate is similar to a uniformly arranged cell of a honeycomb, whereby the specific surface area is increased and hence the heat transfer surface area and the radiation yield.

In the field of infrared radiators a difference is made between light radiators and dark radiators. Light radiators are directly heated by an atmospheric burner and are operated with a suitable fuel such as natural gas or liquid gas. Frequently, they are wall or ceiling-mounted and mostly serve to heat poorly insulated rooms with high ceilings, where their property as an infrared radiation source has advantageous effects, since it is primarily the surfaces exposed to radiation which are heated, whereas the ambient air is heated only in the second line. The name light radiator is due to the visible combustion of a fuel-air mixture on the ceramic burner plate that begins glowing. The ceramic burner plates can reach temperatures of 950° C. and higher.

Dark burners also generate heat by the combustion of an oxygen-fuel gas mixture, but differently from light radiators within a closed radiation tube. The hot gases which are produced through the combustion will heat the surface of the radiation tube which dissipates the heat predominantly as radiation heat. A dark radiator mainly consists of a burner having a burner plate, a fan, a radiation tube and a reflector arranged above the radiation tube. In modern dark radiators the fan is arranged in front of the burner, so that air is forced into the system, whereby a laminar flame distribution is achieved resulting in a uniform heating of the radiation tube. Also, the fan in this construction is not exposed to the hot exhaust gases, which fact clearly reduces the mechanical load on the fan.

The radiation yield in modern dark radiators can amount up to 65%.

Burner plates for infrared radiators are generally known from prior art. The document DE 21 63 498 for instance discloses a burner plate for infrared radiators which comprises recesses arranged on the radiation side and mutually parallel arranged combustion channels for the supply of the fuel-air mixture extending from the mixture side of the plate towards the radiation side, one at least of said combustion channels being concentrically arranged in the ground of the recess and additional ones being distributed over the sides of the recess and to the surfaces between the recesses, which burner plate is characterized in that the combustion channels are distributed over the recesses and the intermediate material webs in a manner such that the flames that are being produced act upon the lateral surfaces of the recess on one side and the intermediate material webs on the other side in such a way that the temperature which is produced in the recesses is almost equal to the temperature produced on the surface of the webs. This construction of the burner plate represents the standard embodiment of the burner plate as employed today.

The document DE 94 02 556 U1 discloses a ceramic gas burner plate which conventionally consists of cordierite. It can be synthetically produced as magnesium aluminum silicate from clay or brickearth, steatite and aluminum oxide.

German patent application DE 44 45 426 A1 discloses a radiant burner having a gas-permeable burner plate, wherein said burner plate can consist in the gas-permeable areas of fiber materials such as e.g. silicon carbide fibers, whereas the gas-impermeable areas are formed of ceramics based on aluminum oxide or cordierite.

The document DE 40 41 061 A1 also discloses a burner plate. The burner plate there described is particularly suited for flat burners and it is based on an aluminum titanate ceramic. Disclosed as particularly suited for the production of corresponding burner plates is an Al₂TiO₅ ceramic.

The document DE 91 16 829 discloses a burner plate for radiant burners which consist for their major part of aluminum oxide.

The aluminum silicates which are used for the production of prior art burner plates have a relatively low burning temperature of max 1000° C., which temperature is within the range of the working temperature of the infrared radiator. This leads to a high load on the burner plates.

Although magnesium silicates like cordierite that are known from prior art have a clearly higher burning temperature of 1300° C., the same must be either burnt from ground and mixed raw materials at high temperatures or pre-burnt masses must be ground and admixed to the burner plate mass and thereafter finished by burning at a low temperature. In addition to a higher material load, this will clearly increase the expenditure at the production of the burner plates.

In view of the above prior art, it is an object of the present invention to provide a ceramic burner plate which is improved with regard to its material.

This object is solved by a ceramic burner plate for infrared radiators, characterized in that the burner plate has a lithium oxide content between 0.63% by weight and 7.6% by weight.

A lithium content within a range between 0.63% by weight and 7.6% by weight corresponds to a lithium-silicate content in the ceramic mass for the burner plate production of 15% at an assumed lithium oxide content within the lithium silicate of 4.2% or 100% of a lithium silicate with an assumed lithium content of 7.6% by weight.

In accordance with the invention, naturally occurring lithium silicates like for instance silicates of the feldspar type petalite represented by the general formula Li₂O*Al₂O₃*8SiO₂ or spodumene represented by the general formula Li₂O*Al₂O₃*4SiO₂ can be used. In accordance with the invention, lithium minerals like lepidolite or also synthetic lithium carbonates can be used in addition.

Besides of the above-mentioned content of lithium oxide the burner plates according to the invention can also include as further constituents at least one oxide from the group consisting of Al₂O₃, SiO₂, Fe₂O₃, TiO₂, CaO, MgO, K₂O, Na₂O, Mn₃O₄, Cr₂O₃, P₂O₅ or ZrO₂.

The ceramic burner plates according to the invention can include the mentioned oxides at amounts stated in the following table.

TABLE 1 Al₂O₃ 22.0-35.0% SiO₂ 55.0-70.0% Fe₂O₃ 0.00-8.00% TiO₂ 0.00-4.00% CaO 0.00-4.00% MgO 0.00-10.0% K₂O 0.00-2.00% Na₂O 0.00-2.00% Mn₃O₄ 0.00-8.00% Cr₂O₃ 0.00-2.00% P₂O₃ 0.00-1.00% ZrO₂ 0.00-5.00% Li₂O 1.00-7.60%

Most expediently, these oxides are added to the material for the production of the ceramic burner plate in the form of suitable minerals.

In accordance with the invention, binder clays including a high content of plastic clay mineral and a high Al₂O₃ content can be used here. Preferably, binder clays are used having an Al₂O₃ content >30% by weight.

In an advantageous manner and according to the invention, binder clays having a low alkali content of <1.5% by weight are used.

The percentage of fine quartz in the advantageously used binder clays amounts to <8% by weight. Moreover, according to the invention, magnesium silicates can be added to the material for the production of the ceramic burner plate.

In a particularly preferred embodiment of the invention a mixture of the aforementioned oxide-containing materials is added to the material for the production of the ceramic burner plate.

The ceramic burner plates according to the invention exhibit a permanent load-carrying capacity at temperatures >1100° C. Moreover, the burner plates according to the invention are not brittle like those known from prior art but they are soft, by which fact their processing is made much easier.

In an advantageous manner the burner plates according to the invention exhibit an extremely low thermal expansion, which fact reduces their mechanical load and additionally facilitates safe binding of the plates at different temperatures to supporting systems. The plates according to the invention are extremely resistant to temperature changes and are extremely durable.

The mechanical hardness of the ceramic burner plates according to the invention can be controlled by means of the burning temperature within a relatively vast range.

For instance, at a burning temperature >1026° C. the ceramic burner plates according to the invention exhibit a standard breaking strength of 16-18 kg. Increasing the burning temperature increases the breaking strength. At a burning temperature of e.g. 100° C. a ceramic burner plate according to the invention exhibits a standard breaking strength of up to 22 kg. By increasing the burning temperature the breaking strength can be increased to clearly more than 24 kg.

Moreover, for the production of the ceramic burner plates according to the invention no pre-burnt raw materials are needed which fact results in clear economic advantages at the plate production.

The following examples are exemplary for the ceramic burner plates according to the invention, whereas the idea on which the invention is based cannot be limited in any way to these examples of execution.

EXAMPLE 1

A ceramic burner plate having a unit weight of 1.2 g/cm⁻¹ and a porosity of 54% has the following composition.

ceramic plate Al₂O₃ 26.17 SiO₂ 65.89 Fe₂O₃ 1.36 TiO₂ 1.05 CaO 0.47 MgO 4.00 K₂O 0.66 Na₂O 0.29 Mn₃O₄ 0.03 Cr₂O₃ <0.01 P₂O₅ 0.08 ZrO₂ <0.01 Li₂O dried sample 1.49 AAS change of weight by −0.03 annealing (1025° C.)

The resistance to temperature changes TWB (1-3) of the ceramic burner plate has been at the value of 1 and it has been determined by means of a quenching test.

There, a TBW value of 1 correlates with a thermal expansion of the ceramic plate of approximately 0.2 at 950° C. 

1. Ceramic burner plate for infrared radiators, characterized in that the burner plate has a lithium content between 0.63% by weight and 7.6% by weight.
 2. Ceramic burner plate according to claim 1, wherein the burner plate includes as further constituents at least one oxide from the group consisting of Al₂O₃, SiO₂, Fe₂O₃, TiO₂, CaO, MgO, K₂O, Na₂O, Mn₃O₄, Cr₂O₃, P₂O₅ or ZrO₂. 